Electrostatic speaker

ABSTRACT

An electrostatic speaker incorporating one and preferably two flexible membranes each with a conductive coating, mounted in a frame comprising clamping surfaces for engaging the membranes and electrical contact means and for maintaining the membranes under tension, with one or more perforated backing plates in spaced position adjacent each membrane but mounted independently of the clamping engagement of the frame with the membranes, and with acoustically transparent spongy cellular damping slabs in pressure engagement with the backing plates.

United States Patent Bobb, deceased [451 Apr. 4, 1972 s41 ELECTROSTATIC SPEAKER 3,496,307 2/1970 Sotome ..179/180 [72] Inventor: Lloyd J. Bobb, deceased, Glenside, Pa. FOREIGN PATENTS 0 APPLICATIONS 1 Assignees: Chester Pond, Doylestown, y 881,584 7/1958 Great Britain ..179/111 Bobb, P Interest to each 1,059,307 2/1967 Great Britain ..l79/1 11 [22] Filed: May 1969 Primary Examiner-Kathleen H. Claffy [21] Appl. No.: 820,888 Assistant Examiner-Thomas L. Kundert Attorney-Synnestvedt & Lechner [52] [1.8. CI. ..l79/lll R 57 ABSTRACT [51] Int. Cl. ..l-l04r 19/02 1 [58] Field of Search ..179/1, 106, 11 1, 180 An l trostatic Speaker incorporating one and preferably two flexible membranes each with a conductive coating, mounted 5 References m in a frame comprising clamping surfaces for engaging the membranes and electrical contact means and for maintaining UNITED STATES PATENTS the membranes under tension, with one or more perforated backing plates in spaced position adjacent each membrane but 1,816,992 8/1931 Vogt ..179/1ll mounted independently of the clamping engagement of the 3l36867 6/1964 Breuenm "'179/111 frame with the membranes, and with acoustically transparent 38389326 6/1968 Peabody-W '179/ spongy cellular damping slabs in pressure engagement with 3,393,764 7/1968 Schaefer ..181/31 the backing plates 1,776,112 9/1930 Edelman ...l79/l 11 3,084,229 4/1963 Selsted et al 179/1 11 6 Claims, 4 Drawing Figures PATENTEDAPR 4 1972 SHEET 1 BF 3 PATENTEDAPR 4 I972 SHEET 2 OF 3 ATTORNEY PATENTEDAIPR 4 m2 3, 654,403

sum 3 OF 3 A g 11450. 0 A 'r'ron NH Y ELECTROSTATIC SPEAKER The present invention is directed to improvements over the type of electrostatic speaker disclosed in my copending application Ser. No. 724,805 filed Apr. 29, 1968, now abandoned.

The invention is concerned with the type of electrostatic speakers in which a flexible membrane carrying a conductive coating is mounted in spaced position adjacent to a substantially rigid perforated backing plate, the signal voltage being applied to the membrane coating and to the backing plate. In this type of electrostatic speaker, the sound is generated by motion of the flexible membrane in relation to the perforated plate and, for fidelity of sound generation, it is desirable that the perforated backing plate be essentially rigid or damped in a manner substantially eliminating motions of the plate in response to the applied signal voltage or in consequence of the sound waves generated by the motion of the membrane.

One of the primary objectives of the present invention is to provide improved damping means for the perforated backing plate of a speaker of the kind above referred to. This is achieved according to the present invention by the employment of a highly cellular slab of plastic or resin material, such as polyurethane foam, which is mounted to have pressure engagement with at least a major portion of the surface of the perforated plate. With such a cellular damping slab, I have found that excellent damping is provided for the perforated plate and, in addition, I have found that the cellular slab is virtually acoustically transparent, so that the side of the speaker assembly at which the perforated plate is mounted may readily be used for generation and propagation of the sound waves originating with the motion of the diaphragm or membrane lying behind the perforated plate.

Because of the above mentioned acoustic transparency of the cellular damping slab, it is possible, according to the present invention, to provide not only for large area surface damping of the backing plate, but also for acoustic coupling of the flexible membrane with another such membrane, also having a perforated plate and a damping slab exposed at the opposite side of the assembly.

A further general objective of the present invention is to simplify the construction of speakers of the kind above referred to, this being achieved, in part, by providing a frame structure which is used to clamp and mount a flexible membrane (or a pair of flexible membranes in the case ofa double ended speaker), the membrane or membranes being pretensioned at the time of assembly. Preferably, where two membranes are used, the clamping of the membranes is effected with the conductive coatings of the membranes presented toward each other and with a contact element between at least a portion of the margins of the membranes. This assembly of frame elements, membranes and contact elements are pre-assembled as a sub-assembly, and the perforated plates and other parts including the damping slabs, and also the strips for establishing separation between the membranes and the perforated plates, are then assembled with said sub-assembly. This greatly facilitates and simplifies construction of speakers of the kind referred to as will appear still more fully after consideration of the accompanying drawings.

The drawings show a preferred embodiment of the present invention in which the speaker includes a pair of acoustically coupled membranes, and in these drawings:

FIG. 1 is an elevational view of an electrostatic speakerincorporating the features of the present invention;

FIG. 2 is an enlarged fragmentary sectional view taken as indicated by the section line 2-2 on FIG. 1;

FIG. 3 is an isometric fragmentary view illustrating on an enlarged scale on one corner of the sub-assembly of frame structure and membranes, but with the various components of the sub-assembly separated from each other, in the manner of an exploded view; and

FIG. 4 is a view of the same type as FIG. 3, but illustrating, in the manner of an exploded view, additional parts of the speaker being brought together with the sub-assembly of the parts shown in FIG. 3.

In the drawings, it will be seen that the frame structure is made up of two counterpart frames 5. In the embodiment illustrated these frames are planar and are square and open in the central area. As best seen in FIG. 3, a series of parts is positioned between the two frames 5-5, including the central perforated contact strips 6, a pair of flexible membranes 7, one located at each side of the contact strips 6. The membranes preferably comprise sheets of resin material such as polyester resin, one form of which is readily available under the Tradename Mylar (E.I. DuPont de Nemours and Co.). The membranes are desirably quite thin, for instance of the order of about one-fourth to one-half mil. Moreover, the membranes are provided with metallic or conductive coatings applied, for instance, by well known vacuum evaporation techniques. Aluminum is an effective metal for coating purposes and an appropriate aluminum coating will be very thin, having a resistance of the order of 3 to 5 ohms as measured across a square of the coated membrane.

The membranes are positioned with their conductive coatings presented toward each other and thus presented so as to engage the contact strips 6. At the outer sides of the membranes gripping strips or gaskets 8 made, for example, of blotting paper are provided. With the contact strips 6 perforated, as indicated, when the parts are clamped together, the gaskets act to press localized areas of the membranes into perforations of the contact strips in the manner indicated in FIG. 2 and thereby provide an interlocking and tight gripping engagement adapted to maintain the membranes in tension. As in the application identified above, I preferably establish a condition of omni-directional tension in the membranes. This may be accomplished by gripping and pulling on edge portions of an oversize piece of a film from which the membranes are to be formed, the essentially uniform pulling force (for instance of about 1% pounds) being effected in a multiplicity of directions radially from the central region thereof. The parts are brought together and clamped between the frames while the tension is maintained in the membranes, so that, in effect, the tension is trapped" by the clamping action of the frames 5-5. Fastening bolts such as indicated at 9 to 10 serve to clamp the frames together with the assembled parts therebetween and thus form a sub-assembly with which other parts may be assembled in the manner described below with particular reference to FIG. 4.

It will be noted that bolts 9 are used at two opposite sides of the frame 5 and that bolts 10 are used at the other two opposite sides. The latter (10) are in the form of studs having threads exposed at both ends and thus projecting at both sides of the sub-assembly for cooperation with other parts to be mounted thereon, as described below.

Turning now to FIG. 4 it will be seen that in the central portion of the figure the sub-assembly comprising the parts illustrated in FIG. 3 are shown in assembled relation, instead of the exploded relation illustrated in FIG. 3. In FIG. 4 the perforated backing plates are indicated at 11 in exploded relation at each side of the central sub-assembly. Between the backing plates 11 and the membranes there are spacer elements 12 formed for example of blotting paper. These may be distributed in any of a variety of patterns over the area of the backing plates, for instance in the form of strips 12 as illustrated and interconnected in a network or windowed" pattern, and further including pips" or small individual or isolated pieces such as indicated at 15. Turned edges or flanges 13 are provided at the perimeter of the marginal strips 12 (see also FIG. 2), and these flanges extend outwardly to overlie the edges of the perforated plates 11, thereby providing for centering the backing plate assembly in the frame opening.

Although only a single backing plate could be employed at each side of the speaker assembly, it is preferred to sectionalize the backing plates and, in the embodiment herein illustrated, each of the backing plates is divided into five sections or strips positioned in edge-to-edge relation, as clearly appears in FIGS. 1 and 4. The adjacent edges of the backing plates are separated from each other and therefore electrically isolated from each other in order that the backing plates may be separately connected with the signal current feed system. In order to retain the backing plates in their desired separated or spaced relation, adhesive strips 14 are applied to the plates in the manner clearly shown in FIGS. 1, 2 and 4. The separator strips 12 if desired may also be coated with an adhesive so that when the backing plates and the separator strips are brought together they will adhere to each other. Similarly, small adhesively coated pieces 15 of separator material such as blotting paper may be applied to the backing plates in the middle of each of the windows" of the separator strip network 12, as is indicated in FIGS. 1,2 and 4.

A set of backing plates and separator parts may be assembled together and then placed within the frame at each side of the speaker assembly, with the separator strips in contact with the membrane at that side.

Slabs of cellular resin material such as indicated at 16 are then applied to the outer faces of the backing plates, and preferably these slabs extend over the entire area of the speaker within the frames 5. The foam slabs are urged into pressure engagement with the backing plates by the fastening devices here shown in the form of metal strips 17 the main portion of each of which lies in the plane perpendicular to the plane of the speaker but each of which has bent and apertured ends 18 cooperating with the projecting threaded ends of the bolts and secured thereto by means of nuts 19. At each connection ofa strip 17 to a bolt 10, an insulating ferrule 20 is applied in order to electrically isolate the strips 17 from the bolts 10. This insulation of the strips 17 from the foam is of advantage for the following reasons. Although the foam is normally an effective insulating material, nevertheless under certain conditions, especially conditions of high relative humidity, the foam may temporarily accumulate sufficient moisture to constitute a leakage path, and as the foam is in contact with the backing plates, and the bolts for mounting the strips 17 are in contact with the conductive surfaces of the membranes, the electrical insulation of the strips with relation to the mounting bolts will avoid leakage between the backing plates and the conductive coatings of the membranes, even under adverse conditions of high relative humidity.

From FIG. 2 it will be seen that the foam slabs 16 (for example seven-eighths inch thick) have contact throughout the area of the perforated plates. The foam slabs in fact apply some pressure to the plates and thereby urge the separator strips 12 into contact with the outer faces of the membranes. The separator strips or spacers at opposite faces of the two membranes are preferably positioned in registry with each other, so that the effect of the pressure applied by the foam slabs is to clamp the membranes together between the separator strips, as clearly appears in FIG. 2. It is not intended and not necessary that this pressure be a heavy pressure, but only sufficient to provide the desired damping action on the plates themselves. A suitable force would be on the order of about 1% pounds at each retaining nut 19. This damping action in the arrangement just described is achieved primarily as a result of the engagement of the foam slabs with the perforated plates, but some damping action is also contributed as a result of the engagement of the edge portions of the perforated plates, and of certain limited strip areas thereof, with the separator or spacer elements 12 and 15.

Although the two membranes of the double ended or pushpull speaker illustrated and described are thus clamped together under the action of the foam slabs on the backing plates and separator means, when the speaker is in use, the membranes separate from each other in substantially all of the area thereof lying between the opposed separator elements 12 and 15. This separation occurs in consequence of the biasing field, provided, for example, by a biasing voltage which is applied across the perforated plates and the conductive coatings on the membranes. The action of the biasing voltage is to draw the adjacent membrane toward the backing plate. This separation of the various membrane areas is indicated in FIG. 2.

With regard to the separation of the membranes under the action of the biasing field, it is to be noted that in effect the space between the membranes is a vented, rather than a sealed space. This is true, notwithstanding the fact that in limited areas the membranes may be brought into contact with each other between the separator strips at the opposite sides of the speaker assembly. This vented space is ensured by providing space, such as indicated in FIG. 3 at 21, between the perforated contact strips which lie between the membranes and which are clamped with the membranes between the frames 5. Since the pressure of engagement of the membranes by the separator strips 12 is not great, and since the separator strips 12 are advantageously formed of a fibrous type of material such as blotting paper, leakage of air between the membranes even in the areas where they are engaged by the separator strips readily occurs. If desired, in order to assure this venting action of the intermembrane space, the separator strips may be provided with embossed grooves such as indicated at 22 in FIG. 4.

The arrangement described above, including the action of the separator strips pressing upon the membranes to bring the membranes close together, and even in contact with each other, is desirable from the standpoint of protecting the electrically conductive coatings which are presented toward each other at the inner faces of the membranes. The substantial restriction of cross sectional vent area is useful in avoiding adverse influences upon the metal coatings. At the same time the fact that the intermembrane space is not sealed is important in securing the desired operation of the speaker and also in providing for free use of the speaker under varying conditions of barometric pressure, as in the cabin of an aircraft. The pressure between the membranes and exteriorly thereof may readily equalize and thus not impair the desired operation of the speaker.

The arrangement described is also of importance because it enables maintenance of a closer spacing between the membranes of a double ended electrostatic speaker than has been practicable heretofore and this reduces the total volume of the air film between the membranes thereby increasing the acoustic coupling between the membranes.

The system of foam slab damping for the backing plates, with the slabs in contact over virtually the entire area of the backing plates, is also highly advantageous for the following reasons. Not only does the foam damping prevent unwanted resonances in the backing plates, but in addition the arrangement of the damping elements with the means for clamping the membranes between the damping slabs at opposite sides of the speaker assembly provides for establishing a more uniform spacing between each membrane and its associated backing plate or plates. This uniformity is of importance because it provides uniform power output over the entire area of the speaker, at the low end of the frequency response range. Nonuniform spacing between the backing plates and the active area of the membranes tends to limit the low frequency power handling ability of the speaker.

As disclosed in my copending application above identified, it is here contemplated that the two speakers incorporated in and acoustically coupled with each other in the assembly, be fed with signal voltages in push-pull fashion. The signal voltage is of course superimposed upon the bias voltage, as is known in the art of electrostatic speakers.

Although only a single backing plate may be provided at each side of the speaker assembly, as was mentioned above, the sectionalizing of the backing plates provides for separate feed of the signal voltage, for instance through a network as described in the copending application above referred to. As shown in FIG. 1, an electrical connection 23 is extended from each of the backing plates, and the network employed in association with the backing plates may be arranged either in the manner shown in the copending application above referred to or in some other desired manner, the signal bias voltage being connected across the network and the connection 25 which is made to one of the contact strips lying between the coated surfaces of the membranes 7.

In connection with the separator strips 12 and the marginal flange 13 provided around the network of separator strips, it is mentioned that this entire windowed separator structure may be stamped out of the central area of the same sheet from which the gaskets 8 are formed. The gaskets 8 (see FIG. 3) are the elements positioned at the outer face of the margins of the membranes in order to provide clamping action against the contact strips 6.

As seen in FIGS. 1, 3 and 4 holes 26 are provided in the corners of the speaker frames 5 and also in various of the intervening parts which are clamped in the frames, these holes being provided as an aid to mounting the speaker, for instance in a speaker cabinet. The corners of the membranes 7, as seen in FIG. 3, do not have holes aligned with the frame holes 26, but it is to be understood that when the parts are assembled and mounting bolts or screws are inserted into the holes 26, the membranes are readily punctured to pass the bolts or screws. The holes provided in the corners for mounting bolts may desirably be provided with insulating sleeves, so that the mounting bolts will not have contact either with the membranes or with the contact strips.

With regard to the foam slabs 16, as indicated above, polyurethane foam is suitable, although it is preferred that the foam employed be of open cell or pore structure, preferably of rather large pore size, with extensive intercommunication between the open pores, so that the foam structure represents substantially nothing more than a skeleton or loose network of relatively thin and fragmentary skeletal strands without intervening membranes or cell walls. In a typical polyurethane foam suitable for the purposes of this invention, the pore count averages 25 pores per linear inch. Although various resin or plastic foam materials can be used, polyurethane has been found particularly effective and, in the form described above has been found to be acoustically transparent to such a degree that its presence was not detected from sound measurements made with and without the interposition of the foam.

A typical polyurethane foam employed in accordance with the invention weighs only about 31 ounces per cubic foot.

Notwithstanding the light weight and highly skeletal character of the foam slabs, it has been found that the damping action applied to the perforated plates is highly effective in eliminating vibrations of the plates, with the consequent elimination of the adverse influences and spurious sounds generated thereby. The fact that the damping material lies between the backing plates themselves at one side, and a series of overlying restraining bars at the other side, increases the efficiency and effectiveness of the damping material because flexure of the backing plates incident to vibration thereof causes compression or sheer action in the damping material rather than mere elongation or contraction of the material in the plane parallel to the backing plates.

The mounting of the perforated plates and the spacers l2, and also of the foam slabs independently of the gripping or clamping of the membranes between the frames 5 is of considerable advantage from the standpoint of production or manufacturing techniques. Since the membranes are initially placed in tension and then clamped between the frames in that condition, the sub-assembly of the frames and membranes may be immediately removed from the equipment and jigs employed for this operation and then the sub-assembly may be brought together with other parts at another work station. In this way the production capacity of the equipment for tensioning and mounting the membranes is increased, because it is not necessary to perform other operations in the equipment for tensioning the membranes and clamping them between the frames.

A further advantage of the frame and membrane assembly is that vibrational modes of the membrane may be examined prior to final assembly, thereby enabling improved quality control.

Thus final performance can be very accurately predicted as the other elements subsequently added are essentially mechanical in nature and therefore only require positional, dimensional and pressure control.

The mounting of the backing plates and of the damping elements for the plates is of advantage not only in the specific embodiment illustrated and described, in which the damping means for each side of the speaker assembly comprises a single slab of cellular material overlying substantially the entire area of the backing plates at that side, but also in arrangements in which damping means comprises elements of lesser extent, i.e., strips or the like which may engage edge portions only of the backing plates.

The arrangement of the invention has the further advantage of providing for ready combining of frame member assemblies which may have different tensioning or which may incorporate membranes of different thicknesses, together with various back plate configurations and spacings, to meet a variety of acoustic requirements.

Still another advantage of the arrangement described incorporating the slabs of foamed material at opposite sides of the speaker is the fact that the foam slabs, although acoustically transparent, in effect, constitute filters which prevent atmospheric dust from reaching the backing plates or the membranes, which are highly charged and therefore tend to attract dust particles.

1 claim:

1. An electrostatic speaker comprising a pair of planar frames, a pair of membranes having conductive coatings presented face to face and having an inter-membrane space, electrical contact means between the membranes, the outer faces of the membranes being exposed within the frames and the membranes being clamped between the frames with the membranes under tension and with the inter-membrane space vented to atmosphere, a pair of electrically conductive perforated planar backing plates overlying the outer faces of the membranes, the membranes and the backing plates being spaced from each other throughout the entire area of the backing plates, spacing elements between each backing plate and the adjacent membrane for maintaining the spacing therebetween, and porous foam slabs at the outer faces of the plates mounted to apply pressure against the plates and thus urge them against each other with the membranes and the spacing elements therebetween.

2. An electrostatic speaker as defined in claim 1 in which the spacing elements for the two backing plates of the pair are arranged in registry with each other at the outer faces of the pair of membranes to thereby provide for clamping of the membranes together in the areas of the spacing elements under the influence of the pressure applied by the foam slabs.

3. An electrostatic speaker as defined in claim 1 in which the foam slabs are acoustically transparent and overlie at least a large part of the area of the perforated backing plates.

4. An electrostatic speaker comprising a tensioned flexible membrane having a conductive coating on at least one face, an electrically conductive substantially rigid and perforated backing plate spaced at one face of the membrane, means for establishing electrical circuit connections with the conductive coating on the membrane and with said plate to provide for connection with a signal source adapted to effect sound generating motion of the membrane with respect to the backing plate, and means for imposing a damping action on the backing plate comprising an acoustically transparent foam slab in engagement with the face of the backing plate presented away from the membrane and spaced elements overlying the foam slab and acting against the slab in a direction perpendicular to and toward the backing plate to establish and maintain pressure engagement of the slab with the backing plate.

5. An electrostatic speaker comprising a tensioned flexible membrane having a conductive coating on at least one face, an electrically conductive substantially rigid perforated backing plate at one face of the membrane, the membrane and the backing plate being spaced from each other throughout the entire area of the backing plate, means for establishing electrical circuit connections with the conductive coating on the membrane and with said plate to provide for connection with a signal source adapted to effect sound generating motion the damping means at one side face of the backing plate comprises separator strips between limited areas of the backing plate and the membrane, and in which the damping means at the other side of the plate comprises a slab of cellular resin material in contact with at least most of the area of the backing plate. 

1. An electrostatic speaker comprising a pair of planar fraMes, a pair of membranes having conductive coatings presented face to face and having an inter-membrane space, electrical contact means between the membranes, the outer faces of the membranes being exposed within the frames and the membranes being clamped between the frames with the membranes under tension and with the intermembrane space vented to atmosphere, a pair of electrically conductive perforated planar backing plates overlying the outer faces of the membranes, the membranes and the backing plates being spaced from each other throughout the entire area of the backing plates, spacing elements between each backing plate and the adjacent membrane for maintaining the spacing therebetween, and porous foam slabs at the outer faces of the plates mounted to apply pressure against the plates and thus urge them against each other with the membranes and the spacing elements therebetween.
 2. An electrostatic speaker as defined in claim 1 in which the spacing elements for the two backing plates of the pair are arranged in registry with each other at the outer faces of the pair of membranes to thereby provide for clamping of the membranes together in the areas of the spacing elements under the influence of the pressure applied by the foam slabs.
 3. An electrostatic speaker as defined in claim 1 in which the foam slabs are acoustically transparent and overlie at least a large part of the area of the perforated backing plates.
 4. An electrostatic speaker comprising a tensioned flexible membrane having a conductive coating on at least one face, an electrically conductive substantially rigid and perforated backing plate spaced at one face of the membrane, means for establishing electrical circuit connections with the conductive coating on the membrane and with said plate to provide for connection with a signal source adapted to effect sound generating motion of the membrane with respect to the backing plate, and means for imposing a damping action on the backing plate comprising an acoustically transparent foam slab in engagement with the face of the backing plate presented away from the membrane and spaced elements overlying the foam slab and acting against the slab in a direction perpendicular to and toward the backing plate to establish and maintain pressure engagement of the slab with the backing plate.
 5. An electrostatic speaker comprising a tensioned flexible membrane having a conductive coating on at least one face, an electrically conductive substantially rigid perforated backing plate at one face of the membrane, the membrane and the backing plate being spaced from each other throughout the entire area of the backing plate, means for establishing electrical circuit connections with the conductive coating on the membrane and with said plate to provide for connection with a signal source adapted to effect sound generating motion of the membrane with respect to the backing plate, and mechanism for mounting the backing plate including means for imposing a damping action on the plate at each of the opposite sides faces thereof, the plate being under pressure engagement between the damping means at the opposite side faces thereof.
 6. An electrostatic speaker as defined in claim 5 in which the damping means at one side face of the backing plate comprises separator strips between limited areas of the backing plate and the membrane, and in which the damping means at the other side of the plate comprises a slab of cellular resin material in contact with at least most of the area of the backing plate. 